Its only stressful in the beginning. You never mentioned what your ammonia levels are at. That is pretty high for nitrates but shouldn't be enough to kill fish. Also your tank is still pretty new, so it could be still cycling, or the rock that you got could have something that was living inside that didn't like the new tank and died in the rock, rotting and causing a spike. Im still pretty new myself, but IMO I would do at least a 50% water change, and test all levels of everything. Usually when one thing is out of whack, others are as well. If your urchin is dying or dead I would remove it from the tank also. I don't think they can poison a tank, but I could also be completely wrong.
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